For generating a stabilized DC output voltage, more and more switching regulators are used for the DC-DC conversion, because of the high efficiency of the switched mode power supplies. New equipment designs and low parts costs with a simple design make the use of switching regulators more pronounced over that of linear regulators. A further advantage of the switching regulator is the increased application flexibility of the output voltage, which can be less than, greater than, or of opposite polarity to that of the input voltage.
When using a monolithic integrated control circuit, only a few circuit parts are required for designing a DC-DC converter. Control circuits of this kind comprise in particular an internal temperature compensated reference voltage, a comparator, a controlled duty cycle oscillator with an active current limit circuit, a driver stage and a high current output switch. A known monolithic control circuit of this kind is for example the integrated circuit MC34063 manufactured by ON Semiconductor Components Industries.
A DC-DC converter of this kind, operating as a down-converter, is shown in FIG. 1. The DC-DC converter comprises an inductor L1, a diode D1 and an integrated control circuit IC1, for providing a stabilized DC output voltage U2. The inductor L1 is in particular a coil. The control circuit IC1 is in this application the integrated circuit MC34063A.
The control circuit IC1 comprises a reference voltage generator RG and a comparator CO for the regulation of the output voltage U2. It comprises further an oscillator OS, having an input terminal 3, to which a capacitor C3 is coupled for defining the oscillation frequency of the oscillator OS. The control circuit IC1 comprises further a logic circuit LC and an output stage with a driver transistor Q2 and an output switch Q1, the transistor Q2 driving the output switch Q1. The comparator CO and the oscillator OS are coupled to the logic circuit LC for operating the output stage.
The control circuit IC1 operates with an input voltage U0, coupled to current inputs 1, 6, 7 and 8 of the control circuit IC1. An overload protection is provided for the DC-DC converter via a low resistance resistor R1 coupled to terminals 6 and 7 of the control circuit IC1. The output switch Q1 is coupled with a current input, terminal 1, to the input voltage U0 and with a current output, terminal 2, to a first terminal of the coil L1 for providing a voltage U1 for the operation of the coil L1.
A freewheeling diode D1 is coupled between ground and terminal 2. The coil L1 is coupled with a second terminal to an output capacitor C2, which provides a smoothing of the output voltage U2. The voltage U2 is coupled via a voltage divider, resistors R3 and R2, and terminal 5 to the comparator CO of the control circuit IC1, for providing a feedback loop FB for the regulation of the DC-DC converter.
The circuit as described above is known and similar circuits are suggested in application sheets of IC manufacturers. Detailed application data for the MC34063 are available by ON Semiconductor Components under HTTP://onsemi.com in a data sheet, Rev. 10, April 2002. According to the data sheet, the maximum permissible switch current for the MC34063 is 1.5 A.
For peak currents higher than 1.5 A, the DC-DC converter has to include an external power transistor, as shown in FIGS. 2 and 3. In FIG. 2 a npn-transistor T1 is coupled with a current input to the input voltage U0 and to the terminal 1 of the integrated circuit IC1. The base terminal of the transistor T1 is coupled to the terminal 2 of the integrated circuit IC1 and the current output is coupled to the inductor L1. The output switch Q1 operates therefore as a driver stage for the transistor T1. The regulation loop of the DC-DC converter, not shown, corresponds to the regulation loop of FIG. 1.
A resistor R4 couples the base terminal of transistor T1 to ground. When the output switch Q1 is blocked, the transistor T1 is blocked also. When Q1 switches through, the transistor T1 switches through respectively, the output switch Q1 providing therefore only the base current for the transistor T1. The current through the transistor T1 is therefore correspondingly higher depending on the current amplification factor of the transistor T1. Therefore, all the current for operating the inductor L1 is provided by the current output of the transistor T1, and hence a power transistor has to be used in accordance with the required output power.
In FIG. 3 a DC-DC converter is shown, which uses a pnp-transistor T2 for providing a higher output power. The current input of the transistor T1 is the emitter, which is coupled in this embodiment to the input voltage U0. The current output, the collector, is coupled to the inductor L1. A resistor R5 couples the base terminal of transistor T2 to the input voltage U0 and a resistor R6 to the terminal 1 of the integrated circuit IC1. The terminal 2 of the integrated circuit IC1 is coupled to ground.
The operation of the DC-DC converter shown in FIG. 3 is similar to the operation of the DC-DC converter of the FIG. 2: When the output switch Q1 of the integrated circuit IC1 switches through, a current flows through the emitter and base of transistor T2 through switch Q1, switching through therefore transistor T2. When Q1 is blocked, the transistor T2 is blocked also, because then the voltage present at the base of transistor Q2 is high.
Corresponding applications for step-up converters and voltage inverting converters are also described in the data sheet for the integrated circuit MC34063.